In the bottling industry, prior art plant for making plastic bottles comprises: a blow-moulding wheel mounted in such a way as to rotate about its longitudinal axis and equipped with at least one moulding unit, which is fed by the blow-moulding wheel around this axis and which is equipped with an interchangeable mould having at least one cavity for blow-moulding a bottle from a respective parison.
The moulding unit also comprises an interchangeable stretching unit comprising stretching rods which are equal in number to the number of moulding cavities of the mould mounted on the moulding unit at a given time, and each of which can move along a predetermined line to engage a respective parison and to axially deform the parison itself.
Prior art moulding units of the above type have several drawbacks due mainly to the fact that substituting the mould means, each time, also substituting the stretching unit, creating serious difficulties for personnel in charge, which in turn means relatively long plant set-up times.
Also known, from patent document FR2795994, is a technical solution which allows the stretching unit to be adapted to moulds with one or, alternatively, two moulding cavities. In this solution, the stretching rods, besides being movable longitudinally in order to interact with the parisons by stretching them, are also movable transversally towards and away from each other. In this way, the stretching rod can be positioned transversally in such a way as to be aligned with the mould.
This solution, however, has the disadvantage of unduly complicating the stretching unit to allow the rod position to be transversally adjusted in the correct manner.
Moreover, this solution makes it difficult to replace a blow-moulding cylinder with one having a different number of moulding cavities (to enable stretching of a different number of parisons).